Since the advent of modern semiconductor circuits, most notably the microprocessor, efforts have been made to design an electronic door lock which provides a secure, "pick-proof" lock that incorporates the advantages offered by a microprocessor. Several such attempts at designing electronic locks are described in U.S. Pat. Nos. 4,573,046; 4,964,023 and 4,031,434. Each of the structures described in the foregoing patents suffers from a common drawback; they cannot be directly utilized within the structures of existing conventional doorlatch locks. Such prior art electronic lock structures generally require new locking hardware to be installed and additional holes to be bored through the door and into the door jamb itself. For example, U.S. Pat. No. 4,573,046, issued to Pinnow, generally discloses an electronic transmitter/receiver locking system wherein the transmitter is preferably located in a watch worn on the user's wrist. The reference does not describe, in other than a conceptual manner, that apparatus which is responsive to a signal receiver located in the door, that would physically actuate the lock mechanism. However, the reference clearly suggests modifying the conventional doorlatch lock hardware so as to implement the locking function. Besides the lack of compatibility with existing door locks, such prior art electronic lock designs suffer other shortcomings.
U.S. Pat. No. 4,964,023, issued to Nishizawa et al. generally discloses an illuminated key wherein the emitted light can be modulated to perform an additional keying function. Presumably, frequency shift keying modulation (i.e., FSK modulation) is utilized, which is easy to duplicate, thereby significantly reducing the security provided by such locking mechanism. Duplication of the FSK modulation "key" may be accomplished, for example, by using a "universal" TV/VCR remote control which has a "learning" function. Duplication can be achieved by simply placing the original "key" in proximity with the "universal" controller and transmitting the key's optical information directly into the controller's sensor.
U.S. Pat. No. 4,031,434, issued to Perron et al. generally discloses an inductively coupled electronic lock that uses a binary coded signal. The key transmits an FSK signal encoded with a preprogrammed code by magnetic induction to a lock unit. The lock unit processes the signal from the key and activates a motor that moves a deadbolt. The power source for both the key and the lock unit is contained in the key. This type of locking device is extremely sensitive to noise and requires fairly close operative proximity between the "transmitter" and the "receiver."
U.S. Pat. No. 4,770,012 issued to Johansson et al., and U.S. Pat. No. 4,802,353 issued to Corder et al. disclose relatively complicated combination type electronic door locks that are partially powered by built-in batteries. The exterior handles of these locks are used to receive user generated entrance codes in a manner similar to mechanical combination locks and use relatively primitive programming schemes. Such lock structures do not use the conventional style doorlatch lock structure but are switched between locked and unlocked states by means of an internal electromagnetic solenoid which retracts an internal pin that allows rotation of the exterior handle and opening of the door. The U.S. Pat. No. 4,802,353 lock also provides for a mechanical key override for the electronic lock structure and can be used with an infrared communication link to activate a remotely located deadbolt lock, of the type described in U.S. Pat. No. 4,854,143. In each of the locks described in these patents, the energy for actually moving the lock latch relative to the door strike plate is provided by the user.
The concept of using an electromagnetic locking device such as disclosed in the above three patents has a number of drawbacks. First, such devices require substantial electrical power since the solenoid electromagnets must remain energized in order to keep the locks in their unlocked states. Accordingly, battery replacement is frequent. For example, U.S. Pat. No. 4,770,012 discloses that the lock battery lasts through roughly 9,000 locking operations, which at a normal door usage rate of 30 operations a day, would be less than a year. U.S. Pat. No. 4,802,353 discloses that the battery lasts 180 days under the same conditions. Second, such electromagnetic devices are also extremely slow. The deadbolt electromagnet in U.S. Pat. No. 4,854,143 requires 8 seconds and 4 seconds respectively to switch to the unlocked and locked states. The door electromagnet disclosed in U.S. Pat. No. 4,802,353 requires four seconds to switch to the unlocked state. Third, the electromagnetic devices which are selected for this application are designed to operate at low currents and cannot resist strong forces along their axes of motion. This means that they cannot be loaded by stiff springs and can be easily tampered with by the application of moderate external magnetic fields. Fourth, in addition to the length of time taken to operate the solenoid, additional time (at least 8 seconds) is required to enter a correct combination code, making the total elapsed time to open a door on the order of 16 seconds. This is much longer than the time required to open a door with a conventional key-operated lock mechanism.
Further disadvantages of the above described electronic combination lock systems are that the entrance code may be visibly detected by others, disabled persons (e.g., blind people) cannot typically use such locks, and those with mechanical overrides features can generally be picked. Also compared to conventional door lock configurations, the above-described combination locks generally require new manufacturing and tooling procedures (as compared to those required for conventional doorlatch locks) and must be partly constructed from nonferrous materials in the vicinity of the electromagnetic device, which limits production options.
What is notably lacking in electronic lock structures heretofore known in the prior art is a simple, "pick-proof" low power lock configuration that is compatible with the internal mechanical locking mechanisms of universally used conventional key-operated doorlatch locks. Such an electronic door lock design would be compatibly usable with, and could readily be designed by lock manufacturers into, existing doorlatch lock structures with a minimum of engineering or production tooling effort or cost. Virtually all existing conventional mechanical lock structures use the rotational motion of a mechanical key about the axis of the key acceptor cylinder to move a locking member. The rotational motion of the key is either directly used to rotate a locking member or is immediately translated into linear motion of a locking member which moves generally along the axis of the key acceptor cylinder. Such simplicity and effectiveness of the conventional mechanical doorlatch locks has not been heretofore duplicated by the complicated, high power consuming or ineffective prior art electronic lock structures.
The present invention addresses the shortcomings of prior art electronic locking structures by using sophisticated low power electronic components to directly replace the mechanical key and key accepting lock cylinder portions of conventional mechanical doorlatch locks while retaining the internal mechanics of such locks for performing the actual door locking functions. Such electronic lock hardware which is designed for compatibility with existing conventional doorlatch locks allows manufacturers' investments in current door lock manufacturing facilities to be retained and takes advantage of state-of-the-art microprocessor-based electronics to control plural lock functions including sophisticated entrance codes, record keeping of authorized entrances, etc.